An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

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Silicon microring resonators

Gradient metasurfaces are two-dimensional optical elements capable of manipulating light by imparting local, space-variant phase changes on an incident electromagnetic wave. These surfaces have thus far been constructed from nanometallic optical antennas, and high diffraction efficiencies have been limited to operation in reflection mode. We describe the experimental realization and operation of dielectric gradient metasurface optical elements capable of also achieving high efficiencies in transmission mode in the visible spectrum. Ultrathin gratings, lenses, and axicons have been realized by patterning a 100-nanometer-thick Si layer into a dense arrangement of Si nanobeam antennas. The use of semiconductors can broaden the general applicability of gradient metasurfaces, as they offer facile integration with electronics and can be realized by mature semiconductor fabrication technologies.

Dielectric gradient metasurface optical elements.

We review photonic applications of dielectric whispering-gallery mode (WGM) resonators-tracing the growth of the technology from experiments with levitating droplets of aerosols to ultrahigh-Q solid state crystalline and integrated on-chip microresonators.

Optical resonators with whispering-gallery modes-part II: applications

Optical fibre nanowires and microwires offer a variety of enabling properties, including large evanescent fields, flexibility, configurability, high confinement, robustness and compactness. These distinctive features have been exploited in a wealth of applications ranging from telecommunication devices to sensors, from optical manipulation to high Q resonators. In this paper I will review the fundamentals and applications of nanowires and microwires manufactured from optical fibres.

https://iopscience.iop.org/article/10.1088/2040-8978/12/4/043001/pdf

Optical fibre nanowires and microwires: a review

Microwires and nanowires have been manufactured by using a wide range of bottom-up techniques such as chemical or physical vapor deposition and top-down processes such as fiber drawing. Among these techniques, the manufacture of wires from optical fibers provides the longest, most uniform and robust nanowires. Critically, the small surface roughness and the high-homogeneity associated with optical fiber nanowires (OFNs) provide low optical loss and allow the use of nanowires for a wide range of new applications for communications, sensing, lasers, biology, and chemistry. OFNs offer a number of outstanding optical and mechanical properties, including (1) large evanescent fields, (2) high-nonlinearity, (3) strong confinement, and (4) low-loss interconnection to other optical fibers and fiberized components. OFNs are fabricated by adiabatically stretching optical fibers and thus preserve the original optical fiber dimensions at their input and output, allowing ready splicing to standard fibers. A review of the manufacture of OFNs is presented, with a particular emphasis on their applications. Three different groups of applications have been envisaged: (1) devices based on the strong confinement or nonlinearity, (2) applications exploiting the large evanescent field, and (3) devices involving the taper transition regions. The first group includes supercontinuum generators, a range of nonlinear optical devices, and optical trapping. The second group comprises knot, loop, and coil resonators and their applications, sensing and particle propulsion by optical pressure. Finally, mode filtering and mode conversion represent applications based on the taper transition regions. Among these groups of applications, devices exploiting the OFN-based resonators are possibly the most interesting; because of the large evanescent field, when OFNs are coiled onto themselves the mode propagating in the wire interferes with itself to give a resonator. In contrast with the majority of high-Q resonators manufactured by other means, the OFN microresonator does not have major issues with input-output coupling and presents a completely integrated fiberized solution. OFNs can be used to manufacture loop and coil resonators with Q factors that, although still far from the predicted value of 10. The input-output pigtails play a major role in shaping the resonator response and can be used to maximize the Q factor over a wide range of coupling parameters. Finally, temporal stability and robustness issues are discussed, and a solution to optical degradation issues is presented.

Optical fiber nanowires and microwires: fabrication and applications

This tutorial describes the transmission, group delay time and quadratic dispersion properties of the four basic building blocs of optical filters built using ring resonators. These building blocs are single-ring resonators in either two-port (all-pass) or four-port (add/drop filter) configuration. The effect of waveguide and coupler loss is included throughout. Explicit expressions to compute the complex amplitude of the circulating wave in the ring, the Q factor, the finesse and the insertion loss are also given. Attention is drawn to the similarities between the ring resonator the Fabry-Pe/spl acute/rot resonator and the Gires-Tournois interferometer. Material properties and fabrication technology are not reviewed.

Transmission, group delay, and dispersion in single-ring optical resonators and add/drop filters-a tutorial overview

Ring-resonator-based optical mirrors are analyzed and their characteristics numerically evaluated. The mirrors are optimized to provide high reflection in a relatively narrow band. A design procedure that relates the required coupling to the bandwidth (BW) and reflection coefficient is presented. The effect of waveguide loss on the transmission and reflection performance is computed. Group delay and dispersion under the influence of loss and circuit-parameter variations are also treated.

Band-limited optical mirrors based on ring resonators: analysis and design

Numerically simulated performance is presented in a new type of bandstop and bandpass filters, built with discontinuity-assisted ring resonator (DARR)-loaded couplers. The operating characteristic of the bandpass filter is similar to that of a long grating with a centered cavity or a photonic crystal with a defect. The filters are tuned by varying the refractive index or the length of the waveguide forming the ring or the cavity. Selectivity and tuning range are controlled by the coupling coefficient of the couplers, by the order of resonance in the rings, and by the reflection coefficient of the discontinuities. A design procedure and the effect of loss are also presented.

All-optical tunable filters built with discontinuity-assisted ring resonators

Results of numerical experiments on several interferometric networks built with 2/spl times/2 couplers are presented. The networks investigated include resonant and nonresonant lattices, generalized Mach-Zehnder (MZ) configurations and transversally coupled spectral filters. Several novel interferometric configurations and novel properties of known configurations are discussed. The numerical method is based on the concept of a universal coupler and on the generalized analysis of interferometric networks. It has been found that relatively simple configurations can provide the very high spectral selectivity needed in some communication applications and the flexibility desired in various sensor applications. Group delay and loss characteristics are also given.

O. Schwelb

Papers

Transmission, group delay, and dispersion in single-ring optical resonators and add/drop filters-a tutorial overview

Band-limited optical mirrors based on ring resonators: analysis and design

All-optical tunable filters built with discontinuity-assisted ring resonators

Characteristics of lattice networks and spectral filters built with 2/spl times/2 couplers